Mr Blair was speaking at the European Bioscience Conference in London.

Protesters want all animal tests banned

He said: "There is a danger, almost unintentionally, that we become anti-science.

"Our conviction about what is natural or right should not inhibit the role of science in discovering the truth. Rather, it should inform our judgement about the implications and consequences of the truth science uncovers."

He said that developments in biotechnology, which could see cures for killer diseases such as cancer, diabetes and heart disease, depended upon research being able to take place.

'No heroes'

"Sometimes it is controversial, as with GM crops or animal testing. Such research is rightly strictly regulated.

"But this government will not tolerate blackmail, even physical assault, by those who oppose it," he said. "To do so would be to give in to intimidation."

He said he had an "open mind" on GM foods and accepted there were "legitimate concerns".

However, he added: "To make heroes of people who are preventing basic scientific research taking place is wrong. It is to substitute aggression for argument."

Stem cell research

Mr Blair said that people had to recognise that in controversial areas such as stem cell research there could be "more than one morally acceptable outcome".

While some people were opposed to all forms of embryo research on ethical grounds, stem cell research had the potential to improve quality of life for those suffering from disease and there were strong ethical arguments in favour of it going ahead, he argued.

There needed to be a "far more considered rational dialogue" between scientists and the public if people were to accept the need for continuing research and development, he added.

He also stressed that biotechnology offered the prospect of massive economic and commercial gains for Britain.

British lead

By 2005, the European biotechnology market is expected to be worth $100bn and employing up to three million people.

The government has gone to great lengths in the past to support the development of GM crops - against the wishes of the majority of people in this country

Peter Riley, Friends of the Earth

While British companies were in the lead in Europe, other countries such as Germany were catching up fast, Mr Blair said.

"Biotechnology is the next wave of the knowledge economy and I want Britain to become its European hub.

"I want to make it clear we don't intend to let our leadership fall behind and are prepared to back that commitment with investment."

Impact on environment

Friends of the Earth said Mr Blair needed to consider the impact of biotechnology, especially genetically modified foods, on the environment.

Spokesman Pete Riley said: "The government has gone to great lengths in the past to support the development of GM crops - against the wishes of the majority of people in this country.

"They need to be sure of what they are doing, and with this new and untested technology they are putting all their eggs in one basket."

But Mr Riley stressed that Friends of the Earth was not "anti-science" and welcomed developments in medicine and other biotechnology fields.

A British group that monitors genetic engineering, Genewatch UK, said Mr Blair didn't understand the public's concerns.

"It isn't an anti-science sentiment. People realise there could be huge benefits from genetic science," said spokeswoman Sue Mayer.

"What the concern is about is whether the government and its institutions can steer the difficult course through the ethical issues, so we get the benefits and not the downside."